1. Field of the Invention
The present invention relates to zoom lenses suitable for use in image pickup apparatuses such as still cameras, video cameras, broadcasting cameras, and digital still cameras.
2. Description of the Related Art
Recently, image pickup apparatuses (cameras), such as video cameras and digital still cameras, including solid-state image pickup devices have become smaller with increased functionality.
Accordingly, demand has increased for small, high-zoom-ratio zoom lenses having a small length and high optical performance over the entire zoom range for use in image-forming optical systems of the image pickup apparatuses.
In a retractable zoom lens having lens units that can be retracted when the camera is not used, it is necessary to reduce the number of lenses included in each lens unit and to reduce the size of each lens unit in order to reduce the overall size of the zoom lens.
In general, the size of the zoom lens can be reduced by reducing the amount of movement of each lens unit during zooming while increasing the refractive power of each lens unit, and reducing the number of lenses included in each lens unit.
However, in the case in which the refractive power of each lens unit in the zoom lens is increased, aberration variation during zooming is also increased. Therefore, it becomes difficult to obtain high optical performance over the entire zoom range and over the entire image plane.
Therefore, to obtain a high zoom ratio and high optical performance while reducing the size of the entire lens system, it is important to adequately set the refractive power of each lens unit and conditions under which each lens unit is moved during zooming.
As an example of a small zoom lens having a zoom ratio of 4.5 or more, a zoom lens which includes four lens units and which performs zooming by moving the lens units is described in U.S. Pat. Nos. 6,853,496, 7,286,298, and 7,193,790. The zoom lens includes a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power arranged in that order from an object side to an image side.
In the zoom lens including four lens units, the second lens unit can be composed of a negative lens and a positive lens to reduce the size of the entire system, as described in U.S. Pat. Nos. 5,134,524 and 6,577,450.
In this type of zoom lens including four lens units, the high zoom ratio and high optical performance cannot be obtained unless the lens structure of the first lens unit, the amount of movement of the first lens unit during zooming, and the lens structure of the second lens unit, which provides a magnification-varying function, are adequately set.